A. Field of the Invention
The present invention relates to a method of manufacturing a monitor pad, especially a method of manufacturing a test pattern for the topographic step height measurement for the CMP planarization process.
B. Description of the Prior Art
Chemical mechanical polishing (CMP) is one of the most important techniques which can provide global planarization for ULSI fabrication, such as 0.3 or 0.25 .mu.m fabrication. In 0.3 or 0.25 .mu.m fabrication, the increasing circuitry miniaturization and density often result in a high degree of varying topography being created on an outer wafer surface during fabrication. Chemical mechanical polishing can effectively provide a substantially planar surface on the wafer surface having varying topography.
The chemical-mechanical polishing process involves holding and rotating a thin, flat wafer of the semiconductor material against a wetted polishing surface under controlled chemical, pressure and temperature conditions. The polishing effect on the wafer results in both a chemical and mechanical action. A particular problem encountered in chemical mechanical polishing is the determination that the surface has been planarized to a desired endpoint. A conventional technique for determining the planar endpoint of a planarized surface is using a monitor pad to reflect the polish condition on the wafer surface.
A monitor pad is a device on a wafer defining for monitoring the polishing condition of the wafer. Since the chemical mechanical polishing process is global polishing, the polishing condition on the surface of the monitor pad shall be equal to the polishing condition on any other area on the wafer surface. Typically, an operator can use an Ellisometer to measure the topographic step height change on the surface of the monitor pad by the reflection or absorption the laser light beam emitting onto the monitor pad. Ellisometer employs maximizing of internal reflection or absorption of laser light into a light transmittable layer of material. The intensity of remaining light emanating outwardly through or from the upper surface is monitored. Thickness and planarity are determinable therefrom.
However, since the laser light beam will be impacted by polysilicon and metal layers, these layers on the monitor pad are usually removed to eliminate noise. Consequently, the height of the silicon oxide layer left will be less than the height of the wafer surface. Therefore, after the planarization process, the thickness removed from the monitor pad will be different from the thickness removed from the wafer surface. Consequently, the Ellisometer cannot get the accurate endpoint measurement from sensoring the area of the monitor pad.
FIGS. 1A and 1B illustrate the situation when the monitor pad and the rest area of the wafer surface are at different heights before the chemical mechanical polishing planarization process. Referring to FIG. 1A, the silicon oxide layer of the monitor pad is of height h 12 while the highest wafer surface is of height H 14 as shown in FIG. 1B. Comparing FIG. 1A and FIG. 1B, it is easy to tell that the surface of the monitor pad is lower than the highest wafer surface. If the polishing endpoint detection relies on the height of the monitor pad, the result will be like the figures as shown in FIG. 2A and FIG. 2B. Referring to FIG. 2A, the surface 22 of the silicon oxide 21 is lower than the highest surface 23 of the wafer as illustrated in FIG. 2B. Consequently, after the chemical mechanical polishing planarization process, the thickness removed H 24 is more than the thickness removed h' 25 of the monitor pad. Nevertheless, the silicon oxide layer remaining on the monitor pad is not significantly different with what it was before. In other words, the thickness difference of (h-h') is not significant enough to determine the real polishing condition of the wafer.